The nature and thickness of the surficial sediments can considerably modify ground motion characteristics, such as amplitude, frequency content and duration. In regions where strong earthquakes are
relatively rare, there is a lack of suitable instrumental data to help understand the effects of local site conditions during future earthquakes. The objective of the present study is to develop a procedure for regional assessment of the local site
effects and to generate ShakeMaps for scenario earthquakes in the St. Lawrence Lowlands. Surficial geology overlying uneven bedrock topography was used as a proxy for seismic site conditions. The amount of ground-shaking amplification due to the
surficial geologic layers is assumed as a function of the thickness and correlated shear-wave velocities. The complex geologic stratigraphy was simplified and grouped into three main units: surface sand, clay, and basal till. A variety of data
including a digital elevation model, borehole data from public databases, existing geological models, and the updated seamless surficial geology map were edited in ArcGIS, and then imported into gOcad. A 3-D grid was built to determine the thickness
values of each unit throughout the study area. Approximately 150 km of high resolution shear wave reflection seismic sections, 40 downhole seismicmeasurements, and 700 seismic refraction and reflection site evaluations were examined to determine
horizontal and vertical shear wave velocity profiles of unconsolidated sediments. Both average shear wave velocity-depth and interval velocity-depth functions were developed and correlated to the main surficial units. Ground motion prediction
equations are currently being combined with shake type analyses to identify areas of potentially intensive ground shaking. The generated ShakeMaps will be used at a later stage to assess potential damage and losses with the recently adapted Hazus
software.

Summary

(Plain Language Summary, not published)The nature and thickness of the sutficial sediments can considerably modify ground motion characteristics, such as amplitude, frequency content and
duration. The objective of the present study is to develop a procedure for regional assessment of the local site effects and to generate ShakeMaps for scenario earthquakes in the St. Lawrence Lowlands. Surficial geology overlying uneven bedrock
topography was used as a proxy for seismic site conditions. The complex geologic stratigraphy was simplified and grouped into three main units: surface sand, clay, and basal till. A 3-D grid was built to determine the thickness values of each unit
throughout the study area. Both average shear wave velocity-depth and interval velocity-depth functions were developed and correlated to the main surficial units.